Not very much at all.
A spring like this is relatively inexpensive and decent quality.
http://www.airsoftparts.ca/store2/in...roducts_id=884
If you are mechanically inclined you can find a description of how to change the spring online and save yourself the gun smithing cost of the springs installation. However if you are not so good with such things its worth your money to invest in having someone skilled do it for you.